1. Technical Field
The invention relates to induction heating and an improved induction furnace. More particularly, the invention relates to an induction furnace for melting materials not susceptible to inductive heating at lower temperatures but which are susceptible to inductive heating at higher temperatures, especially upon melting. Specifically, the invention relates to an induction furnace having an electrically conductive susceptor disk which is inductively heated whereby heat is transferred from the disk to such materials to make them susceptible to inductive heating whereby the materials are then inductively heated to melt them.
2. Background Information
Induction furnaces are well known in the art. However, there are a variety of difficulties related to the inductive heating and melting of materials that are initially non-conductive or which have particle sizes sufficiently small so that they are not susceptible to inductive heating. Many prior art induction furnaces utilize a conductive crucible such that an induction coil couples with the crucible to transfer energy directly to the crucible to heat the crucible. Heat is then transferred from the crucible to the material to be melted via thermal conduction. In certain cases, the induction frequency and the thickness of the crucible wall may be selected so that a portion of the electromagnetic field from the coil allows coupling with any electrically conductive material inside the crucible to inductively heat the material directly. However, the direct inductive heating in such cases is quite limited. Because direct inductive heating of the material to be melted is far more effective than the method described above, a system to effect such direct inductive heating is highly desirable.
In addition, the conductive crucibles of the prior art may react with the material to be melted which causes unwanted impurities in the melt and thus requires the use of a non-reactive liner inside the crucible to prevent formation of such impurities. Typically, such liners are electrically non-conductive and thermally insulating. As a result, the transfer of heat from the crucible to the materials to be melted is greatly impeded, thus substantially increasing melting times. To expedite the transfer of heat from the crucible to the material to be melted, the crucible must be heated to undesirably high temperatures which can decrease the life of the crucible and liner.
In addition, there remains a need for an induction furnace capable of producing a continuous melt in an efficient manner, especially for semi-conductor materials. An efficient continuous melt induction furnace is particularly useful for continuous formation of semi-conductor crystals, which are highly valued in the production of computer chips.
U.S. Pat. No. 6,361,597 to Takase et al. teaches three embodiments of an induction furnace especially intended for melting semi-conductor materials and adapted to supply the molten material to a main crucible for pulling of semi-conductor crystals therefrom. Unlike the prior art discussed above, Takase et al. uses a quartz crucible which is electrically non-conductive along with a susceptor which is in the form of a carbon or graphite cylinder. In each of the three embodiments of Takase et al., the carbon or graphite cylinder susceptor is initially inductively heated by a high frequency coil whereby heat is transferred from the susceptor to raw material inside the crucible in order to begin the melting process. Once the raw material is melted, it is directly inductively heated by the high frequency coil in order to speed up the melting process. While this is a substantial improvement over the previously discussed prior art, the induction furnace of Takase et al. still leaves room for improvement, as discussed below.
The first embodiment of Takase et al. involves the use of a pipe extending upwardly into the quartz crucible whereby the pipe receives molten material from within the crucible by overflow and transmits it to a main crucible from which semi-conductor crystals are pulled. The carbon cylinder susceptor encircles the quartz crucible and is moveable in a vertical direction. Prior to melting the material in the crucible, the carbon cylinder is positioned so it covers the entire side wall of the crucible. Once some of the material is melted, the carbon cylinder is moved upwardly so that the molten material is inductively heated by the coil. Once the raw material is fully melted, additional raw material is added and the carbon cylinder is moved downwardly to cover the upper half of the side wall of the crucible so that the carbon cylinder is inductively heated and transfers heat therefrom to aid in melting the added raw material.
While the first embodiment of Takase et al. permits the susceptor to be substantially removed from the electromagnetic field of the induction coil so that it is not further inductively heated or so that the inductive heat is minimized therein, this process still has some disadvantages. One disadvantage to this configuration is the need to provide a mechanism to move the cylindrical susceptor upwardly and downwardly. Another disadvantage of the configuration is the need for a mechanism to monitor the melt in order to determine the proper time to move the susceptor away from the crucible side wall. Because direct inductive heating of the molten materials is more effective than inductive heating of the susceptor and subsequent transfer of heat from the susceptor to the material, any time that the susceptor is left in place after the molten material is susceptible to inductive heating, it prevents the more efficient direct inductive heating of the melt.
The second embodiment in Takase is similar to the first embodiment except that the pipe for transferring molten material from the quartz crucible to the main crucible does not extend upwardly into the quartz crucible. A mass of the initial raw material is disposed over the opening of the pipe and effectively serves as a stopper until the stopper portion is itself melted. In order to prevent the stopper from being melted too soon, the carbon cylinder initially only covers about two thirds of the upper portion of the side wall of the crucible so that heat transferred from the carbon cylinder is transmitted only to about the upper two thirds of the raw material. As the raw material is melted, the carbon cylinder is moved downward to cover the entire side wall of the crucible. Then the carbon cylinder is moved upwardly to cover the upper half of the side wall of the crucible whereby continued inductive heating of the carbon cylinder allows heat transfer from the carbon cylinder to raw material that is added to the melt. Induction heat is also generated in the melt at this point.
The second embodiment similarly suffers from the need for moving the cylindrical susceptor in a vertical fashion. The process must also be monitored in order to determine when to move the susceptor cylinder downwardly to maintain a reasonably high efficiency. Further, the susceptor interferes with the inductive heating of the molten material when positioned around the crucible while there is still unmelted raw material within the crucible.
In the third embodiment, Takase et al. provides a pipe which extends upwardly into the crucible as in the first embodiment to provide overflow of the molten material to the main crucible. In this embodiment, the susceptor has a crucible-like configuration whereby the susceptor cylindrical portion covers the sidewall of the quartz crucible and the bottom of the susceptor covers the lower surface of the quartz crucible. In this embodiment, the susceptor is not vertically moveable. Instead, the thickness of the susceptor sidewall and the frequency applied by the coil are selected so that the penetration depth of the induction current will extend beyond the susceptor into the quartz crucible so that it can inductively heat material inside. As with the prior embodiments, the susceptor is inductively heated and then transfers heat to the raw material to begin the melting process. Once the melting process has begun, inductive heating of the melt also occurs and the melt continues as a result of both inductive heating directly of the molten material as well as transferred heat from the inductively heated susceptor. In addition, the frequency applied to the coil is preferably initially at a relatively high frequency and then once the melting has begun is shifted to a relatively low frequency to better focus inductive heating of the molten portion of the material.
This third embodiment primarily suffers from the fact that the cylindrical susceptor remains in place and thus prevents inductive heating from being focused more effectively on the raw material within the crucible. Instead, the coil continues to inductively heat the carbon cylinder so that energy which might be applied to the material is absorbed by the carbon cylinder, which transfers heat to the raw material in the crucible in a far less effective manner.